DISTINCT EFFECTS OF OMEGA-TOXINS AND VARIOUS GROUPS OF CA2-ENTRY INHIBITORS ON NICOTINIC ACETYLCHOLINE-RECEPTOR AND CA2+ CHANNELS OF CHROMAFFIN CELLS()

The effects of omega-toxins and various Ca2+ antagonist subtypes on th e Ca-45(2+) entry into bovine adrenal medullary chromaffin cells stimu lated via nicotinic acetylcholine receptors or via direct depolarizati on with K+, have been compared. The conditions selected to stimulate t he Ca-45(2+) entry consisted of a 60-s period of exposure of cells to 100 mu M of the nicotinic acetylcholine receptor agonist dimethylpheny lpiperazinium or to 70 mM K+. The N-type voltage-dependent Ca2+ channe l blockers omega-conotoxin GVIA and MVIIA (1 mu M) inhibited Ca-45(2+) entry stimulated by dimethylphenylpiperazinium or K+ by around 25-30% . The P-type Ca2+ channel blocker omega-agatoxin IVA(10 nM) did not af fect the dimethylphenylpiperazinium nor the K+ responses; 1 mu M (Q-ch annel blockade) inhibited both responses by around 50%. The N/P/Q-type Ca2+ channel blocker omega-conotoxin MVIIC (1 mu M) inhibited the Kevoked Ca-45(2+) entry by 70%, while dimethylphenylpiperazinium was bl ocked by 50% (P <0.001). The L-type Ca2+ channel blockers nifedipine, furnidipine,diltiazem or verapamil (3 mu M each) inhibited much more t he dimethylphenylpiperazinium than the K+ response. The dimethylphenyl piperazinium signal was blocked 71, 88, 89, and 53%, respectively, by nifedipine, furnidipine, diltiazem and verapamil, and the K+ response by 38, 29, 22, and 10%. Combined omega-conotoxin MVIIC (1 mu M) and fu rnidipine (3 mu M) blocked 100% of the K+ evoked Ca-45(2+) entry. Howe ver, combined omega-conotoxin GVIA (1 mu M), and furnidipine left unbl ocked 50% of the K+ response. The 'wide spectrum' Ca2+ channel antagon ists flunarizine or dotarizine (3 mu M each) blocked the dimethylpheny lpiperazinium and the K+ responses to a similar extent (50%); cinnariz ine (3 mu M) inhibited more the dimethylphenylpiperazinium (82%) than the K+ response (21%). At 3 mu M, the highly lipophilic beta-adrenocep tor antagonist(+/-)-propranolol, reduced by 68% the dimethylphenylpipe razinium signal and by 23% the K+ signal. Other high lipophilic beta-a drenoceptor antagonists such as metoprolol and labetalol, reduced litt le the dimethylphenylpiperaziniun and the K+ responses. The highly lip ophilic agent perfluridol blocked the dimethylphenylpiperazinium respo nse by 30% and the K+ response by 50%. One of the least lipophilic com pounds tested, (+)-lubeluzole, blocked by 40% the dimethylphenylpipera zinium and the K+ responses. These data are compatible with the idea t hat the various omega-toxin peptides used to separate pharmacologicall y the different voltage-dependent Ca2+ channels expressed by neurones, do not block the neuronal nicotinic acetylcholine receptor ion channe l. In contrast the L-type Ca2+ channel blockers do block the nicotinic acetylcholine receptor ionophore. Lipophilicity of the compounds is n ot a requirement for Ca2+ channel or nicotinic acetylcholine receptor blockade.